Third-Generation Partnership Project (3GPP) Long Term Evolution (LTE) is a new standard mobile network technology that uses only packet-switched (PS) protocols, including at the radio or air interface. It has been posited that the simplified architecture of LTE systems will reduce operating expenses for mobile service providers. Increasingly, mobile service providers are integrating LTE networks with pre-existing second-generation (2G) and third-generation (3G) networks that utilize a combination of circuit-switched (CS) and packet-switched protocols, such as Global System for Mobile Communications (GSM), General Packet Radio Service (GPRS), Enhanced Data Rates for GSM Evolution (EDGE), and Universal Mobile Telecommunications System (UMTS) networks. For example, both GSM and UMTS networks utilize circuit-switched protocols for voice services.
Mobile service providers may find it difficult to gracefully transition to LTE technologies because they must continue to support legacy 2G/3G hardware in which the service provider or its customers invested, even after new LTE infrastructure is deployed. For example, service providers will need to continue to support customers who have legacy 2G/3G mobile devices that are not capable of interacting with LTE radio access networks. Additionally, service providers may continue to maintain their 2G/3G radio access networks for some period of time before fully replacing them with LTE radio access networks, since these legacy networks represent a substantial prior capital investment.
In existing approaches, the legacy circuit-switched core network architecture must be maintained to support voice calls conducted using legacy 2G/3G mobile devices and 2G/3G radio access networks. Furthermore, the 3GPP has suggested that in order to interface LTE subsystems with 2G/3G subsystems, voice calls between a mobile device on an LTE subsystem and a mobile device on a 2G/3G subsystem should be circuit-switched by the circuit-switched core network of the 2G/3G subsystem. The suggested approach would require that mobile service providers indefinitely maintain two separate core networks: the legacy circuit-switched core network and a packet-switched core network. Maintaining two separate core networks leads to increased operating and capital expenses. Service providers could realize savings in their operating and capital expenses if 2G/3G circuit-switched core network hardware could be gracefully decommissioned.
One previous approach to eliminate a circuit-switched core network is described in U.S. Patent Application Publication No. 2004/0029615, titled “Transmission of Voice Over Packet-Switched Systems,” PCT No. PCT/EP01/11164, filed Sep. 10, 2001, the entirety of which is incorporated herein by reference. In the previous approach, transmission of voice over Internet Protocol (IP) is performed in a UMTS system by using a hybrid mode of attach where the speech bearer path from the mobile device is transported to the network controller in circuit-switched mode and from there onwards in packet-switched mode. The control signaling from the mobile device is sent over IP to the core network. Unfortunately, in the previous approach, the mobile device must be capable of generating and interpreting session initiation protocol (SIP) signaling messages, e.g., via an installed SIP architecture or application. Thus, unmodified 2G/3G mobile devices that do not include a SIP application may not utilize the previous approach.